This invention relates generally to connectors for optical fibers, and more particularly, to methods for aligning single-mode and multi-mode optical fibers using graded refractive index (GRIN) lenses. The need for optical fiber connectors in optical fiber communication systems and other applications has long been apparent. Ideally connectors should present only a minimal loss in the fiber transmission medium.
The principal causes of loss in all fiber optical connectors are angular and spatial misalignment. These alignment problems are particularly acute for single-mode fiber connectors, which have extremely small dimensional tolerances. In order to butt-couple two single-mode fibers with less than 0.1 decibel (dB) loss, the fiber cores must be aligned to a precision of less than one micron (1.times.10.sup.-6 meter).
One approach to the alignment of single-mode fibers is to employ a graded refractive index lens on each side of the connector, and to butt the lenses together instead of the fibers themselves. Commercially available lenses for this purpose are sold under the name SELFOC lenses. (SELFOC is a registered trademark of Nippon Sheet Glass Co.) These lenses will be referred to in this specification as graded refractive index lenses, or GRIN lenses.
A quarter-pitch GRIN lens functions analogously to a collimating lens. The refractive index of the lens material varies across its cross section in such a manner as to expand a very small source of light, emerging from a single-mode fiber, into a much broader, parallel beam. If a second quarter-pitch GRIN lens is placed adjacent to the first one, the parallel beam is focused down to almost a point focus, for launching into a single-mode fiber in the second connector half. This approach has the advantage of greatly reducing the requirements for lateral alignment of the fibers, i.e. the required tolerance for lateral fiber alignment is greater. However, the use of connector lenses requires extreme precision of angular alignment. For a connector loss of 0.1 dB, the angular alignment tolerance is 0.0003 radians, or approximately one minute of arc (1/60 of a degree). Therefore, the GRIN lens connector approach trades dimensional alignment-tolerance for angular alignment tolerance, and there is, therefore, a need for an accurate and convenient method of angular alignment of the connector lenses.
One cannot rely on precision manufacture of the GRIN lenses, since not all such lenses are perfect plane cylinders, and losses or part wastage will inevitably result. In the past, techniques for assuring precision in the connector halves have relied on there being a near-perfect "master" parallel beam, generated either from a perfect connector half or from a separate source. The procedure typically used is to align and orient each manufactured connector half with the parallel beam. This may not always be possible for some lens components, and can still lead to wastage. The fiber is then positioned and attached to the lens, and in theory any two connector halves that have been matched to the master parallel beam will be perfectly matched to each other.
Unfortunately, these prior-art techniques have not always worked in practice, and there is a need for an alternative approach to angular alignment of GRIN lens connector halves. The present invention is directed to this end.